Impellers for fluid handling apparatus of the rotary positive displacement type



May 14, 1963 w. E. ROSE 3,089,638

IMPELLERS FOR FLUID HANDLING APPARATUS OF THE ROTARY POSITIVEDISPLACEMENT TYPE Filed Dec. 1, 1958 3 Sheets-Sheet 1 INVENTOR W/LL/S E.ROSE BY QM, 9 m- ATTORNEYS SPLACE YPE Filed Dec.- 1, 1958 3 Sheets-Sheet2 May 14, 1963 w. E. ROSE IMPELLERS F OR FLUID HANDLING APPARATUS OF THEROTARY POSITIVE DI MENT T INVENTOR WILL/.5 E. Ross,

ATTORNEYS May 14, 1963 w. E. ROSE 3,089,638

IMPELLERS FOR FLUID HANDLING APPARATUS OF THE ROTARY POSITIVEDISPLACEMENT TYPE INVENTOR WILL/3 E. ROSE BY Qa/mm, WW $501222 ATTORNEYSUnited 3,089,638 IMPELLERS FOR FLUID HANDLING APPARATUS ggPfi'zlHEROTARY POSITIVE DISPLACEMENT Willis E. Rose, Conuersville, Ind.,assignor to Dresser Industries, Inc., Dallas, Tex., a corporation ofDelaware Filed Dec. 1, 1958, Ser. No. 777,350 8 Claims. (Cl. 230-441)This invention relates to fluid handling apparatus of the rotarypositive displacement type, and is particularly directed to theprovision of improved forms of lobed impellers for use in such devices.

Inasmuch as the invention is especially well adapted for embodiment inimpellers for blowers of the wellknown Roots type, the followingdisclosure will be directed primarily to this specific application ofthe inventive concept. By so doing, however, it is not intended to limitthe scope of the invention to blowers because it will be obvious that ithas equal utility in rotary positive exhausters, boosters, pumps andcompressors.

The intermeshing impellers used in Roots type blowers have acharacteristic shape comprising a central hub or waist portion which isfixed to a rotatable shaft and two or more lobe portions of identicalcontour or profile which extend radially outwardly from the waist. Theconvex profiles of the impeller lobes heretofore used have varied inform from circular arcs to curves of involute or cycloidal design, whilethe profiles of the waist sections between lobes have been concavelycurved in different ways so as to receive and form substantiallyfluidtight seals with the lobes of mating impellers as the impellersrotate.

The impeller shapes of the prior art are diiiicult to construct and aresubject to various disadvantages depending upon their specific profiles.For example, when a pair of mating impellers of the commonly usedinvolute type rotate in cooperate relationship, they form trappedpockets which render such impellers unsuitable for handling fluidshaving entrained incompressible liquids such as sealing water, and whichalso impede performance when the blower is used as a vacuum booster dueto the carry-over volumes which must be re-expanded. Cycloidal typeimpellers, on the other hand, make only single point contact as theyrotate without the formation of trapped pockets, but their performanceis limited from the standpoint of displacement.

The principal object of the present invention is to provide an improvedform of lobed impeller for use in Roots type blowers and similarapparatus which is so contoured as to have practically only pointcontact with its mating impeller at all positions of relative rotation,thereby avoiding the formation of trapped pockets simi larly to acycloidal impeller, and to still maintain a relatively largedisplacement comparable to that of an involute impeller.

Another object is the provision of impellers of the character describedhaving profiles of novel configuration which improve the noisecharacteristics and decrease the power requirements of rotary positivedisplacement devices in which they are incorporated.

A further object is to provide a relatively simple method ofestablishing the profiles and producing templates of impellers havingthe characteristics sought by the present invention.

These and other objects of the invention will appear more fully uponconsideration of the detailed description of the embodiments of theinvention which follows. In this connection, although only two specificforms of impellers and one method of establishing impeller profiles aredescribed and illustrated in the accompanying draw- 3,039,638 PatentedMay 14, 1963 ice ings, it is to be expressly understood that thesedrawings are for purposes of illustration only and are not to beconstrued as defining the limits of the invention, for which latterpurpose reference should be had to the appended claims.

In the drawings, wherein like reference characters indicate like partsthroughout the several views:

FIG. 1 is a diagrammatic sectional view of a Roots type blower embodyinga pair of two-lobe impellers having profiles of the novel form providedby the present invention, this view showing the impellers in what willbe termed the position, i.e., with the tip or central point of the lobeof one impeller contacting the central point of the waist of the otherimpeller, and indicating the geometry of the method of establishing thecurvature of the waist portions of the impellers;

FIGS. 2 and 3 are diagrams indicating the relative positions of the twoimpellers of FIG. 1 and their single contact points when the impellershave turned through angles of 22 /2 and 45, respectively, from the 90position;

FIGS. 4, 5, 6, 7 and 8 are diagrammatic plan views illustrating onemethod of generating the convex curves of the lobe portions of theimpellerprofiles and of constructing templates for the manufacture ofimpellers embodying the present invention; and

FIGS. 9, l0 and 11 are diagrammatic views similar to FIGS. 1, 2 and 3,respectively, of a blower comprising a pair of three-lobe impellersembodying the invention.

In general, an impeller constructed in accordance with the presentinvention may be described as having a Waist portion defined by concavecircular arcs the ends of which are substantially coincident with thepitch circle of the timing gear fixed to the impeller shaft, while theconvex profiles of the lobe portions are continuous non-circularconjugate curves generated from the waist profiles of the matingimpeller. The arc defining each segment of the impeller waist portionsubtends an angle at the axis of the impeller shaft equal to 36O/2n,where n is the number of lobes, i.e., 90 in the case of a two-lobeimpeller and 60 in a three-lobe impeller. I In order to attain theimproved results sought by the present invention, the width or thicknessof the impeller waist should be maintained within the range of fromabout /3 to about /2 the gear pitch diameter for two-lobe impellers, andfrom about /2 to about /3 the gear pitch diameter for three-lobeimpellers.

As will be explained hereinafter, the radius of the circular arcdefining the surface of each Waist segment of impellers constructed inaccordance with the present invention may be determined by the formula360 PD W 2 360 PD 2 [COS 4n [5113 Rw: PDW

wherein R is the impeller waist radius, n is the number of impellerlobes, PD is the gear pitch diameter and W is the waist width orthickness. The dimensions R PD and W for twoand three-lobe impellers areindicated in FIGS. 1 and 8, respectively. The curve defining the convexsurface of each lobe, however, is not readily susceptible of definitionby formula, but may be generated in a relatively simple manner as laterdescribed with reference to FIGS. 4-8.

Referring now to FIG. 1, the device diagrammatically illustrated thereinis a Roots type blower comprising a casing or cylinder 21 having twosemicylindrical wall portions laterally separated to provide a fluidinlet 22 at the bottom, an outlet 23 at the top and a pumping chamber 24in the central portion of the casing, and a pair of intermeshingimpellers 25 and 26 of identical shape which are mounted on parallelshafts 27 and 28 and are adapted to be rotated thereby in oppositedirections at a constant velocity ratio through a pair of timing gears,the pitch circles of which are indicated in broken lines at 29 and 30.The shafts 27 and 28 are coaxial with the semicylindrical wall portionsof casing 21 and are so spaced laterally that, when the impellers are inthe 90 position indicated in FIG. 1, the central point of the profile ofone lobe of one impeller is in single point contact with the centralpoint of the profile of one side of the waist of the other impeller.

It will be understood, of course, that each of the impellers iselongated in a direction perpendicular to the plane of FIG. 1 and has auniform cross-sectional shape of the form shown in the figure. In theinterest of simplicity, the description herein will refer to the lineswhich define the sectional contour or profile of the impellers, ratherthan to the surfaces of which said lines are the axial projections.

It will also be understood that, in referring to con-tact between themating impellers, it is not intended to imply that the impellers mustactually touch one another, because at the point of contact betweenimpellers constructed in accordance with the present invention there isusually an accurately gauged clearance on the order of .017". The termcontact as used herein should therefore be construed to include thesituation wherein there is a small working clearance between the closestpoints of the impellers at all positions of relative rotation.

The central or Waist portion 31 of each impeller surrounding theassociated shaft is bounded by a pair of circular arcs 32 of identicalradius, the ends of which arcs are substantially coincident with pitchcircles 29 and 30 and are connected by convex curves 33 which define theouter ends or lobe portions 34 of the impeller. Curves 33 are so formedthat, as the impellers rotate in opposite directions as indicated by thearrows in FIG. 1, at each instant the active lobe of one impellercontacts the waist of the other at only one point. For example, when theimpellers have rotated 22 /2 from the 90 position of FIG. 1 to thatillustrated in FIG. 2, the contact between the lobe of impeller 26 andthe waist of impeller 25 is still at a single point, but the point hasmoved clockwise on the lobe and counterclockwise on the waist from thecentral point shown in FIG. 1. Rotation through another 22 /2 to the 45position of FIG. 3 shifts the contact further along the impellerprofiles, clockwise in the case of impeller 26 and counterclockwise onimpeller 25, to the points where the waist arc of impeller 25 joins thelobe curve thereof and where the lobe curve of impeller 26 joins thewaist arc thereof.

The radius to be used for the waist arcs 32 of impellers constructed inaccordance with the present invention may be determined geometrically inthe manner indicated in FIG. 1. As shown, the first step is to draw aline OY outwardly from the center of pitch circle 29 in a horizontaldirection, i.e., collinear with the centers of both pitch circles, andthen located a point B on line OY at a distance from center 0 equal to/2 the selected Waist width, i.e., W/2. As previously indicated, thewidth of the waist (W) of a two-lobe impeller should be not less thanabout /3 the gear pitch diameter (PD), in order to insure adequatemechanical strength of the impeller and shaft, and not more than about/2 the gear pitch diameter, in order to provide adequate displacement.Preferably, the waist width of a two-lobe impeller is made equal toabout the gear pitch diameter.

The next step is to draw two additional radial lines OZ and OZ makingangles of 45 with line OY and intersecting pitch circle 29 at points Cand D, respectively. The center of a circular are passing through thepoints C, B and D may then be determined in well known manner by drawingthe line CB and erecting a perpendicular at the midpoint F thereof, thepoint A where said perpendicular intersects line OY being the center.The circular are defining the waist profile may then be struck using enBF BE By reference to FIG. 1, it will also be seen that BC= /BE +CE andCE=sin 45 CO Substituting these values for BC, BF and BE in Equation 2gives (cos 45 COBO) +(sin t5XC' 0) (3) 2(c0s 45XCO-BO) In Equation 3,the terms CO and B0 are the radius of the gear pitch circle and /2 thewaist width, respectively, and are therefore known values, PD/2 and W/2,respectively. The angle 45 may also be expressed as 360/ 4n, where n isthe number of lobes of the impeller, because, as previously stated, theare 32 defining each side of the waist of any two-lobe impellerembodying the present invention subtends an angle of at the axis of theimpeller shaft, i.e., the point 0. Substituting these known values inEquation 3, the formula for waist radius AB, i.e., R becomes 0 eos( X PDW The convex curves 33 defining the lobe profiles of impellers embodyingthe invention may be generated from the waist arcs 32 by the methodillustrated in FIGS. 4-8, which are also illustrative of a method ofmaking templates to be used in machining the impellers.

The first step is to obtain two rectangular metal plates 35 and 36 ofsuitable size and thickness, only one of which is shown in FIG. 4, andto describe on each plate the perpendicular axes XX and Y-Y, and two 45lines a-a and bb, all passing through the point 0 which represents theimpeller axis. On each plate is also described, from point 0 as acenter, a circle having a diameter equal to PD, the diameter of the gearpitch circle of the timing gears with which the impellers are to beused. The points B are then marked on axis Y--Y at distances from point0 equal to W/Z, i.e., /2 the desired waist width. Points A are nextmarked on the Y--Y axis at distances from points B equal to the waistradius R as determined in accordance with formula 1. From points A ascenters the waist arcs CBD are then described.

In order to facilitate the operations hereinafter described, each plateis provided With a pair of drilled holes 37 located on axis XX onopposite sides of point 0 which serve to receive bolts or screws forsecuring the plates to machining fixtures and rotating gears. Each plateis also marked with lines 38 to indicate the corner portions of theplate which should be removed for clearance purposes during the lobecurve generating procedure.

After machining off the corners of each plate down to the lines 38, themetal between waist arcs CBD and the upper and lower edges of the plateis removed, and the waist profiles defined by arc CBD are accuratelyground. At the completion of these operations, each of plates 35 and 36will be in the form shown in FIG. 5, it being understood that all of theabove-described operations are performed on each plate. I

Plates 35 and 36 are next mounted on a rotating gear fixture 39 withplate 35 on top of and just touching plate 36, and with the YY axis ofplate 35'aligned with the XX axis of plate 36, as indicated in FIG. 6.Gauge blocks may be used for support and proper spacing of the plates,and bolts or screws may be inserted in holes 37 in order to hold theplates in proper position on the gear fixture. It will "heunderstoodthat the gears of fixture 39, portions of which are visiblethrough the machined-out waist portions of the plates, have the samepitch diameter as the impeller timing gears. In order to insure thatplates 35 and 36 are properly mounted relatively to one another whenfixed to the gear fixtures, the plates should be rotated. through 45 tothe position shown in FIG. 7 and checked to see that the 45 lines b-b ofboth plates fall into alignment. In this position, one end of one of thewaist profile arcs of plate 35 is directly above the point on the pitchcircle of plate 36 which defines one end of one of the Waist profilearcs of the latter.

The lobe curves of plate 36 may be generated by tracing a great seriesof fine marked lines from the waist profiles of top plate 35 onto bottomplate 36, turning the gears of fixture 39 to a different position foreach tracing. For example, with the plates in the. 45 position of FIG.7, the portion of the arc defining the right-hand side of the waist ofplate 35 which overlies the portion of plate 36 outside the pitch circlethereof, i.e., from point D to point G, is traced onto the surface ofplate 36. The gears are then turned through a small angle so as torotate plates 35 and '36 in counterclockwise and clockwise directions,respectively, and the tracing operation is repeated, again following thewaist curve of plate 35 from point D in a counterclockwise direction. Bycontinuing this tracing operation while the gears are rotated in smallincrements through a total angle of 90, i.e. until point C of theright-hand waist curve of plate 35 becomes coincident with point C ofthe lower curve of those portions of the traced lines lying closest tothe pitch circle will produce a clearly visible profile for the lobe ofplate 36, as indicated in FIG. 8.

After turning the gears through another 90 in the same direction, thetracing operation may be repeated beginning with coincidence of thepoint D of the waist curve at the left-hand side of plate 35 (as viewedin FIG. 7) and the corresponding point D of the upper waist curve ofplate 36. When the two series of lines have thus been traced, the faceof plate 36 will have an appearance similar to-that illustrated roughlyin FIG. 8, from which it can be seen that the generated lobe profilesare readily discernible. It will be understood, of course, that FIG. 8does not shown as many tracings as would be made in practice.

Plates 35 and 36 are then removed from gear fixture 39 and reassembledwith plate 35 in the position previously occupied by plate '36, and viceversa, whereupon the tracing procedure above described is repeated,using the waist curves of plate 36 to generate the lobe profiles ofplate 35.

After both plates have been so marked, those portions of the profiletracings lying closest to the pitch circles may be punch marked and thegenerated lobe curves may be faired through the punch marks. The platesare then machined and ground down to the generated lines to providefinished templates having the contours illustrated 6 in FIG. 1, usablein known manner for machining of the impeller castings.

Referring now to FIGS. 9411, the blower shown therein represents anotherembodiment of the present invention as applied to three-lobe impellers.As illustrated, blower casing 41, having a fluid inlet 42, outlet 43 andpumping chamber 44, houses a pair of impellers 45 and 46 which aremounted on and rotatable by shafts 47 and 48, respectively, and aresynchronized by timing gears indicated by the pitch circles 49 and 50.

Each of impellers 45 and 46 has a waist portion 51 defined by threecircular arcs 52 of identical radius, the ends of which arcs aresubstantially coincident-with pitch circles 49 and 50 and are connectedby convex curves 53 which form the profiles of the lobe portions 54 ofthe impeller. Curves 53 are so formed that, as the impellers rotate inopposite directions as indicated by the arrows in FIG. 9, one lobe ofone impeller contacts one of the waist segments of the other at only asingle point. For example, when the impellers have rotated 15 from theposition shown in FIG. 9 to that illustrated in FIG. 10, the contactbetween the lobe of impeller 46 and the waist of impeller 45 remains ata single point, although the point has moved clockwise on the lobe andcounterclockwise on the waist from the central point shown in FIG. 9.Rotation through another 15 to the 30' position of FIG. 11 results inshifting of the point of contact further along the impeller profiles, inthe same directions as those just mentioned, to the points where thewaist arc of impeller 45 joins the lobe thereof and where the lobe curveof impeller 46 joins the waist arc thereof.

The radius of each of waist arcs 52 may be determined geometrically inthe same manner as that previously de scribed with reference to thetwo-lobe embodiment of FIG. 1, except that each of radial lines OZ andOZ from the center 0 of pitch circle 50 makes an angle of 30, instead of45, with horizontal line OY. The length of the waist radius for thethree-lobe impellers of FIG. 9 may be determined mathematically by thesame Formula 1 as that previously discussed in connection with thetwo-lobe embodiment of the invention.

Likewise, the convex curves 53 which define the contours of lobes 54 maybe generated from the waist arcs 52 by a procedure similar to thatpreviously described and illustrated in FIGS. 4-8.

There is thus provided by the present invention an improved form oflobed impeller for use in fluid handling apparatus of the rotarypositive displacement type which combines the advantages, with aminimization of the disadvantages, of the various types of impellersheretofore used. Impellers constructed in accordancewith this inventionprovide a continuous, substantially'single contact point of action whicheliminates trapped pockets, and at the same time provide greaterdisplacement than those impellers of the prior art which arecharacterized by point contact of the mating impellers. Incomparisonwith the commonly used involute form, the impellers of the presentinvention not only are capable of handling incompressible fluids, butalso deliver more cubic feet of fiuid per horsepower per minute and arequieter in operation. The invention also provides a relatively simplemethod of developing the profiles of impellers having the desiredcharacteristics, and of producing said profiles in the form oftemplates.

Although two specifically different forms of impellers embodying theinvention and one procedure for developing their profiles have beendescribed and illustrated herein, it will be obvious that the inventionis not limited to these illustrative examples, but is capable of avariety of mechanical and procedural embodiments. Reference is thereforeto be had to the appended claims for a definition of the limits of theinvention.

What is claimed is:

1. Fluid handling apparatus of the rotary positive displacement typecomprising a pair of lobed impellers mounted on parallel shafts androtatable at a constant velocity ratio through timing gears fixed tosaid shafts, each of said impellers having a central Waist portionadapted to be mounted on one of said shafts and a plurality ofidentically shaped lobe portions extending radially outwardly from saidWaist portion, the profile of each of said impellers consisting of alike plurality of identical concave circular arcs of uniform radiusdefining said waist portion and a like plurality of identical convexnoncircular curves defining said lobe portions, each of said circulararcs subtending an angle at the axis of the impeller shaft equal to360'/2n, where n is the number of said lobe portions, and each of saidnon-circular curves being generated from a circular are identical withthose defining said waist portion so that, when said impellers arerotated in opposite directions at the same velocity through said timinggears, there is substantially point contact only between said impellersat all positions of relative rotation.

2. Fluid handling apparatus as defined in claim 1 wherein the thicknessof the waist portion of each of said impellers is between about /3 andabout /3 the pitch diameter of the associated timing gear, and the endsof each of the circular arcs defining said waist portion aresubstantially coincident with the pitch circle of said timing gear.

3. In a fluid handling device of the rotary positive displacement type,a pair of lobed impellers mounted on parallel shafts and rotatable at aconstant velocity ratio through timing gears fixed to said shafts, eachof said impellers having a central waist portion and twoidenticallyshaped lobe portions extending in diametrically opposite directions fromsaid waist portion, the profile of each of said impellers consisting oftwo concave circular arcs of uniform radius defining said waist portionand two convex non-circular curves defining said lobe portions, thethickness of said waist portion being between about /3 and about /2 thepitch diameter of said timing gears, each of said circular arcssubtending an angle at the axis of the associated shaft equal to 90, andeach of said noncircular curves defining the lobe portions of eachimpeller being generated from a circular are identical with thosedefining said Waist portion so that, when said pair of impellers arerotated in meshing relationship through said timing gears, there issubstantially point contact only between said impellers at all positionsof relative rotation.

4. In a fluid handling device of the rotary positive displacement type,a pair of lobed impellers mounted on parallel shafts and rotatable at aconstant velocity ratio through timing gears fixed to said shafts, eachof said impellers having a central waist portion and three identicallyshaped lobe portions extending radially outwardly from said waistportion, the profile of each of said impellers consisting of threeconcave circular arcs of uniform radius defining said waist portion andthree convex noncircular curves defining said lobe portions, thethickness of said Waist portion being between about /t and about /3 thepitch diameter of said timing gears, each of said circular arcssubtending an angle at the axis of the associated shaft equal to 60, andeach of said non-circular 8 curves defining the lobe portions of eachimpeller being generated from a circular arc identical with thosedefining said waist portion so that, when said pair of impellers arerotated in meshing relationship through said timing gears, there issubstantially point contact only between said impellers at all positionsof relative rotation.

5. For use in fluid handling apparatus of the rotary positivedisplacement type having a pair of lobed impellers mounted on parallelshafts and rotatable at a constant velocity ratio through timing gearsfixed to said shafts, an impeller having a central waist portion coaxialwith the associated shaft and timing gear and a plurality of identicallyshaped lobe portions not exceeding three in number extending radiallyoutwardly from said Waist portion, the profile of said impellerconsisting of a like plurality of concave circular arcs of uniformradius defining said waist portion and a like plurality of convexnon-circular curves defining said lobe portions, each of said circulararcs subtending an angle at the axis of said associated shaft equal to360l2n where n is the number of said lobe portions, each of saidnon-circular curves being generated from a circular are identical withthose defining said waist portion, the radius of each of said circulararcs being so related to the pitch diameter of the timing gears, thethickness of the waist portion and the number of said lobe portions thatwhere R is the arc radius, n is the number of said lobe portions, PD isthe gear pitch diameter and W is the waist thickness and lies in therange of from about PD/ 3 to about 2PD/ 3.

6. An impeller as defined in claim 5 wherein n is equal to 2 and W isbetween about PD/3 and about PD/2.

7. An impeller as defined in claim 5 wherein n is equal to 2 and W isequal to about 3PD/ 8.

8. An impeller as defined in claim 5 wherein n is equal to 3 and W isbetween about PD/Z and about 2PD/ 3.

References Cited in the file of this patent UNITED STATES PATENTS166,295 Palmer et al. Aug. 3, 1875 587,907 Ames et al Aug. 10, 18971,442,018 Wendell Jan. 9, 1923 1,676,202 Isom et al July 3, 19281,769,153 Meyer July 1, 1930 1,798,059 Bilgram et a1. Mar. 24, 19312,578,187 Hill et al Dec. 11, 1951 FOREIGN PATENTS 10,128 Great Britainof 1884 282,752 Great Britain May 31, 1928 566,077 Belgium Apr. 15, 1958861,733 Germany Jan. 5, 1953 953,289 Germany Nov. 29, 1956 1,170,967France Sept. 29, 1958

1. FLUID HANDLING APPARATUS OF THE ROTARY POSITIVE DISPLACEMENT TYPECOMPRISING A PAIR OF LOBED IMPELLERS MOUNTED ON PARALLEL SHAFTS ANDROTATABLE AT A CONSTANT VELOCITY RATIO THROUGH TIMING GEARS FIXED TOSAID SHAFTS, EACH OF SAID IMPELLERS HAVING A CENTRAL WAIST PORTIONADAPTED TO BE MOUNTED ON ONE OF SAID SHAFTS AND A PLURALITY OFINDENTICALLY SHAPED LOBE PORTIONS EXTENDING RADIALLY OUTWARDLY FROM SAIDWAIST PORTION, THE PROFILE OF EACH OF SAID IMPELLERS CONSISTING OF ALIKE PLURALITY OF IDENTICAL CONCAVE CIRCULAR ARCS OF UNIFORM RADIUSDEFINING SAID WAIST PORTION AND A LIKE PLURALITY OF IDENTICAL CONVEXNONCIRCULAR CURVES DEFINING SAID LOBE PORTIONS, EACH OF SAID CIRCULARARCS SUBTENDING AN ANGLE AT THE AXIS OF THE IM-PELLER SHAFT EQUAL TO360*/2N, WHERE N IS THE NUMBER OF SAID LOBE PORTIONS, AND EACH OF SAIDNON-CIRCULAR CURVES BEING GENERATED FROM A CIRCULAR ARC IDENTICAL WITHTHOSE DEFINING SAID WAIST PORTION SO THAT, WHEN SAID IMPELLERS AREROTATED IN OPPOSITE DIRECTIONS AT THE SAME VELOCITY THROUGH SAID TIMINGGEARS, THERE IS SUBSTANTIALLY POINT CONTACT ONLY BETWEEN SAID IMPELLERSAT ALL POSITIONS OF RELATIVE ROTATION.